Fluorinated polymers including polyvinylidene fluoride (PVDF) and ethylene-chlorotrifluoroethylene copolymers (ECTFE) are widely used in the preparation of microfiltration and ultrafiltration membranes due to their good thermal stability, chemical resistance, excellent processability and convenience in controlling the porosity and the morphology of the membranes thereby provided.
Membranes are typically manufactured by using solutions of fluorinated polymers in suitable solvents. According to known techniques, a clear polymer solution is precipitated into two phases: a solid, polymer-rich phase that forms the matrix of the membrane, and a liquid, polymer-poor phase that forms the membrane pores. Polymer precipitation from a solution can be achieved in several ways, such as cooling, solvent evaporation, precipitation by immersion in water, or imbibition of water from the vapour phase. If precipitation is rapid, the pore-forming liquid droplets tend to be small and the membranes formed are markedly asymmetric. If precipitation is slow, the pore-forming liquid droplets tend to agglomerate while the casting solution is still fluid, so that the final pores are relatively large and the membrane structure is more symmetrical.
In these techniques, it remains nevertheless key to provide for stable and homogeneous solutions of fluorinated polymers as starting materials.
The vast majority of fluorinated polymers can be readily dissolved in suitable solvents to form stable solutions. These solvents include N-methylpyrrolidone (NMP), N,N-dimethylacetamide (DMAc), N,N-dimethylformamide (DMF), dimethyl sulphoxide (DMSO) and phthalates.
With regards to NMP, DMF and DMAc, which have been since years the solvents of choice in the industry for solution-based hollow-fiber spinning processes in the manufacture of PVDF membranes, these solvents are now facing environmental and safety concerns, having regards to the safety risks associated to their handling and to possible leakage/emissions in the environment, so questing for substitution.
For instance, NMP has been notably classified according to the European regulation (EC) No1272/2008 in the hazard class Repr.1B code H360D (may damage the unborn child), Eye Irrit.2 code H319, STOT SE 3 code H335, Skin Irrit.2 H315 and according to the European directive 67/548/EEC it is classified as Reprotoxic Cat2 code R61, Xi codes R36/37/38. Further more it is submitted to the Toxic Release Inventory (SARA Title III Section 313).
Similarly, DMAc is covered by index number 616-011-00-4 of Regulation (EC) No 1272/2008 in Annex VI, part 3, Table 3.1 (the list of harmonised classification and labelling of hazardous substances) as toxic for reproduction category 1B (H360D: “May damage the unborn child”). The corresponding classification in Annex VI, part 3, Table 3.2 (the list of harmonised and classification and labelling of hazardous substances from Annex I to Directive 67/548/EEC) of Regulation (EC) No 1272/2008 is toxic to reproduction category 2 (R61: “May cause harm to the unborn child”).
Also, DMF has been classified as toxic to reproduction category 1B (H360D: “May damage the unborn child”) according to Regulation (EC) No 1272/2008 and is included in Annex VI, part 3 (index number 616-001-00-X), Table 3.1 (list of harmonised classification and labelling of hazardous substances). The corresponding classification in Annex VI, part 3, Table 3.2 (the list of harmonised classification and labelling of hazardous substances from Annex I to Directive 67/548/EEC) of Regulation (EC) No 1272/2008 is toxic to reproduction category 2 (R61: “May cause harm to the unborn child.”).
The present invention thus provides a solution for obviating to environmental and safety concerns which arise in using NMP, DMF, DMAc, phthalates or other similar solvents and provides an alternative process for manufacturing membranes.